Flange for semiconductor die

ABSTRACT

A semiconductor package includes a curved body and a plurality of semiconductor die. The curved body includes first and second opposing end regions and an intermediate center region. The curved body has a first inflection point at the center region, a second inflection point at the first end region and a third inflection point at the second end region. The center region has a convex curvature with a minimal extremum at the first inflection point, the first end region has a concave curvature with a maximal extremum at the second inflection point and the second end region has a concave curvature with a maximal extremum at the third inflection point. The plurality of semiconductor die are attached to an upper surface of the curved body between the maximal extrema.

BACKGROUND

Most power semiconductor packages use copper or alloyed copper heatsinks such as CuMoCu or CuW as the flange upon which power semiconductordie are attached. A lid is typically attached to the flange to encloseand protect the die. During the die attach, lid attach and other packageassembly processes, the metallic flange typically becomes bowed. Thebowing occurs as a result of CTE (coefficient of thermal expansion)mismatches between the semiconductor die and the metal flange, forexample. Attaching a bowed flange to a circuit board results in poorthermal performance because the flange and circuit board are not in goodsurface contact with one another. Instead, there is a gap between theflange and the circuit board. Having a gap between the flange and thecircuit board significantly decreases thermal performance of the overallassembly, which is particularly problematic for high power applications.Materials such as thermal grease and indium solder foils have been usedto fill the gap between a bowed flange and a circuit board to which theflange is fastened. However, thermal performance still suffersappreciably even with the use of gap filler materials since the leastrestrictive thermal pathway is direct contact between the flange and thecircuit board.

SUMMARY

According to an embodiment of a flange, the flange comprises a curvedbody having first and second opposing end regions and an intermediatecenter region. The curved body has a first inflection point at thecenter region, a second inflection point at the first end region and athird inflection point at the second end region. The center region has aconvex curvature with a minimal extremum at the first inflection point,the first end region has a concave curvature with a maximal extremum atthe second inflection point and the second end region has a concavecurvature with a maximal extremum at the third inflection point.

According to an embodiment of a semiconductor package, the semiconductorpackage comprises a curved body including first and second opposing endregions and an intermediate center region. The curved body has a firstinflection point at the center region, a second inflection point at thefirst end region and a third inflection point at the second end region.The center region has a convex curvature with a minimal extremum at thefirst inflection point, the first end region has a concave curvaturewith a maximal extremum at the second inflection point and the secondend region has a concave curvature with a maximal extremum at the thirdinflection point. A plurality of semiconductor die are attached to anupper surface of the curved body between the maximal extrema.

According to an embodiment of a method of manufacturing a substrateassembly, the method includes providing a substrate having a relativelyflat surface and placing a curved body on the relatively flat surface ofthe substrate. The curved body includes first and second opposing endregions curved in a first direction facing the substrate and anintermediate center region curved in a second direction facing away fromthe substrate. The curved body has a plurality of semiconductor dieattached to an upper surface of the curved body facing away from thesubstrate. The method also includes bolting down the end regions of thecurved body to the substrate so that a lower surface of the curved bodyflattens out and contacts the relatively flat surface of the substrateover a length of the curved body after bolting down.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an embodiment of a flange having a curved bodyfor attaching semiconductor die.

FIG. 2 is a side view of the flange of FIG. 1.

FIG. 3 is a side view of an embodiment of a semiconductor packageincluding a plurality of semiconductor die attached to an upper surfaceof the curved flange body of FIG. 1.

FIG. 4 is a side view of the semiconductor package of FIG. 3 with a lidattached to the upper surface of the curved flange body.

FIGS. 5-7 are side views of an embodiment of a substrate assemblyincluding the semiconductor package of FIG. 3 during different phases ofbeing attached to a substrate.

DETAILED DESCRIPTION

FIG. 1 illustrates a top-down plan view of an embodiment of a flange 100for attaching semiconductor die. The flange 100 comprises a singlecontinuous metallic curved body 102 including first and second opposingend regions 104, 106 and an intermediate center region 108. The flange100 can be made from any type of metallic material such as copper or acopper alloy. For example, the flange 100 may comprise C19210 (K80),C19400, CuMoCu, CuW or any other suitable type of metallic material. Theends 110, 112 of the flange 100 have respective inlets or openings 114,116 for receiving a bolt to fasten the flange 100 onto another componentsuch as a circuit board (not shown in FIG. 1). The curved body 102 isintentionally curved prior to die attachment. The amount of curvature isa function of several variables, e.g. such as the die attach processparameters (e.g. temperature, duration, etc.), the lid attach processparameters (e.g. temperature, duration, etc.), parameters associatedwith other processes, the flange material, the die material, etc. Thecurvature imparted on the curved body 102 is selected so that the flange100 becomes relatively flat when subsequently bolted down onto asubstrate such as a heat sink or circuit board as will be described inmore detail later herein. In one embodiment, the flange 100 is stampedto impart the desired curvature.

FIG. 2 is a side view of the flange 100, and illustrates an embodimentwhere the curved body 102 has a subtle M-shape. Imparting an M-shapedcurve to the curved body 102 yields very good mechanical contact betweenthe bottom surface 118 of the flange 100 and a substrate (not shown inFIG. 2) after bolt down. To achieve the final desired curvature justprior to substrate mounting, the flange 100 is bowed with a specificinitial curvature which depends upon the materials, processes and theprocessing sequence utilized.

In more detail, the curved body 102 has a first inflection point 120 atthe center region 108, a second inflection point 122 at the first endregion 104 and a third inflection point 124 at the second end region106. The center region 108 of the curved body 102 has a convex curvaturewith a minimal extremum at the first inflection point 120, the first endregion 104 has a concave curvature with a maximal extremum at the secondinflection point 122 and the second end region 106 has a concavecurvature with a maximal extremum at the third inflection point 124.

Stamping the flange 100 to curve the body 102 leaves crimp marks 126 onthe bottom and upper surfaces 118, 128 of the flange 100 at the minimalextremum 120 and the maximal extrema 122, 124 as shown in FIG. 1. Thecurved body 102 can be curved so that the minimal extremum 120vertically extends beyond the ends 110, 112 of the flange 100 by adistance represented by y-x in FIG. 2. In some embodiments, x is in therange of 1 to 3 mils and y is in the range of 3 to 5 mils. In anembodiment, the ends 110, 112 of the flange 100 point downward toincrease flange-to-circuit-board surface contact during assembly.

FIG. 2 shows the concave curvatures of the end regions 104, 106 having asmaller radius (R2) than the convex curvature (R1) of the center region108. In one embodiment, the radius of the concave curvatures is about ⅙the radius of the convex curvature. For example, R2 can be about 5inches and R1 about 30 inches for a 40 mil thick flange. In someembodiments, the flange 100 is about 40 to 50 mils thick and the curvedbody 102 has a peak-to-peak height from the minimal extremum 120 to themaximal extrema 122, 124 of about 2 to 4 mils as represented by distancey in FIG. 2. Generally, the flange 100 can have any desired width,length and thickness. The curvature radii (R1, R2) and the peak-to-peakheight (y) of the curved body 102 are a function of the flangedimensions, the materials, the processes and the processing sequenceutilized as described above. The ideal flange curvature is modeled as afunction of these parameters and/or other parameters that cause flangebowing during processing, and thus depends on several variables. Afterthe desired curvature is imparted on the flange 100, semiconductor diecan be attached to the flange 100.

FIG. 3 illustrates an embodiment of a semiconductor package 300including a plurality of semiconductor die 302 attached to the uppersurface 128 of the curved body 102. The semiconductor die 302 areattached to the curved body 102 between the maximal extrema 122, 124.The die 302 can be attached using any suitable type of die attachmaterial 304 such as solder, epoxy, etc. For example, the die attachmaterial 304 may be AuSn, AuSi, AuGe, etc. Other die attach materialsmay be used. The die attach process imparts some bowing on the curvedbody 102 due to the die attach temperature, duration, die attachmaterial, etc. However, the curved body 102 still maintains the sameoverall curved shape imparted prior to die attach. Bowing caused duringthe die attach process is considered when selecting the initialcurvature to impart on the flange 100 as described above.

FIG. 4 illustrates the semiconductor package 300 after an optional lid306 is attached to the upper surface 128 of the curved body 102 betweenthe maximal extrema 122, 124. The lid 306 encloses the semiconductor die302. The lid 306 may be attached to the curved body 102 using an epoxysuch as polyimide, or any other suitable material. A ceramic windowframe that surrounds the die 302 may also be attached to the uppersurface 128 of the curved body 102 between the lid 306 and the flange100 for providing electrical connections. The lid and window frameattach processes impart additional bowing on the curved body 102 due tothe lid attach temperature, duration, lid attach material, etc. However,the curved flange body continues maintains the same overall curved shapeimparted prior to lid and die attach. Bowing caused during the lid andwindow frame attach processes is also considered when selecting theinitial curvature to impart on the flange 100 as described above. Thesemiconductor package 300 is ready for attachment to a substrate such asa heat sink, a circuit board, a heat plug component of a circuit board,etc.

FIG. 5 illustrates an embodiment of a substrate assembly 500 during aninitial phase of attaching the semiconductor package 300 to a substrate502 such as a heat sink, a circuit board, a heat plug component of acircuit board, etc. The package 300 is shown without the optional lid306 for ease of illustration. The upper surface of the substrate 502 isrelatively flat. If the substrate 502 is a circuit board, the region ofthe circuit board to which the curved body 102 is to be mounted mayinclude a copper or copper alloy heat slug for added heat dissipation.During the initial attachment phase, the convex curved center region ofthe curved body 102 is placed in contact with the substrate 502. Thatis, the first inflection point 120 at the center region 108 of thecurved body 102 is placed in contact with the substrate 502. The ends110, 112 of the curved body 102 point longitudinally outward or downwardtoward the substrate 502, but not upward. This way, the ends 110, 112 ofthe curved body 102 provide good downward pressure during flangeattachment, forcing the curved body 102 to flatten out during boltingdown. If the flange ends 110, 112 were to point upward away from thesubstrate 502, a lifting movement could occur during bolting down whichwould create an undesirable gap between the bottom surface 118 of thecurved body 102 and the substrate 502. Bolts 504, 506 are placed throughthe inlets 114, 116 formed in each end 110, 112 of the curved body 102for attaching the curved body 102 to the substrate 502.

FIG. 6 illustrates the substrate assembly 500 during an intermediatephase of attaching the semiconductor package 300 to the substrate 502.During the intermediate attachment phase, the bolts 504, 506 are screwedinto the substrate 502. In response, the curved body 102 begins toflatten out. Particularly, the ends 110, 112 of the curved body 102provide downward pressure which forces the curved body 102 to flattenout during bolting down. As the curved body 102 continues to flatten,more of the bottom surface 118 of the curved body 102 comes into contactwith the substrate 502.

FIG. 7 illustrates the substrate assembly 500 during a final phase ofattaching the semiconductor package 300 to the substrate 502. During thefinal attachment phase, the bolts 504, 506 are screwed into their finalposition in the substrate 502, causing the curved body 102 to becomeessentially flat. The entire bottom surface 118 of the curved body 102is in direct contact with the substrate 502. As such, there isessentially no void between the curved body 102 and the substrate 502.Thus, no gap filler material is needed. Having essentially the entirebottom surface 118 of the curved body 102 in direct contact with thesubstrate 502 greatly enhances thermal performance of the substrateassembly 500.

In addition, the curved body 102 is elastic. As such, the curved body102 returns to a curved shape after removal from the substrate 502. Thatis, the curved body 102 is made of a material that returns to itsoriginal shape after the stress which caused the flange body to deform(i.e. flatten out) is removed. Accordingly, the curved body 102 returnsto its original curved shape in response to the bolts 504, 506 beingremoved from the substrate 502.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper” and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

1. A method of manufacturing a substrate assembly, comprising: providinga substrate having a relatively flat surface; placing a curved body onthe relatively flat surface of the substrate, the curved body includingfirst and second opposing end regions curved in a first direction facingthe substrate and an intermediate center region curved in a seconddirection facing away from the substrate, the curved body having aplurality of semiconductor die attached to an upper surface of thecurved body facing away from the substrate; and bolting down the endregions of the curved body to the substrate so that a lower surface ofthe curved body flattens out and contacts the relatively flat surface ofthe substrate over a length of the curved body after bolting down. 2.The method of claim 1, wherein the opposing end regions of the curvedbody point downward toward the substrate prior to bolting down.
 3. Themethod of claim 1, wherein the lower surface of the curved body is indirect contact with the relatively flat surface of the substrate afterbolting down.
 4. The method of claim 1, wherein the substrate is acircuit board.
 5. The method of claim 1, further comprising stamping asingle continuous metallic body to form the curved body with the centerregion having a convex curvature with a minimal extremum at a firstinflection point, the first end region having a concave curvature with amaximal extremum at a second inflection point and the second end regionhaving a concave curvature with a maximal extremum at a third inflectionpoint.
 6. The method of claim 5, comprising stamping the singlecontinuous metallic body so that the minimal extremum extends verticallybeyond opposing ends of the curved body.
 7. The method of claim 5,comprising stamping the single continuous metallic body so that theopposing ends of the curved body point downward toward the substrate. 8.The method of claim 5, comprising stamping the single continuousmetallic body so that each concave curvature has a smaller radius thanthe convex curvature.
 9. The method of claim 8, comprising stamping thesingle continuous metallic body so that the radius of each concavecurvature is about ⅙ the radius of the convex curvature.
 10. A method ofmanufacturing a substrate assembly, comprising: providing a substratehaving a relatively flat surface; placing a curved body on therelatively flat surface of the substrate, the curved body includingfirst and second opposing end regions curved in a first direction facingthe substrate and an intermediate center region curved in a seconddirection facing away from the substrate, the curved body having aplurality of semiconductor die attached to an upper surface of thecurved body facing away from the substrate; and fastening the curvedbody to the substrate so that a lower surface of the curved bodyflattens out and contacts the relatively flat surface of the substrateover a length of the curved body after fastening.
 11. The method ofclaim 10, wherein the opposing end regions of the curved body pointdownward toward the substrate prior to fastening.
 12. The method ofclaim 10, wherein the lower surface of the curved body is in directcontact with the relatively flat surface of the substrate afterfastening.
 13. The method of claim 10, wherein the substrate is acircuit board.
 14. The method of claim 10, further comprising stamping asingle continuous metallic body to form the curved body with the centerregion having a convex curvature with a minimal extremum at a firstinflection point, the first end region having a concave curvature with amaximal extremum at a second inflection point and the second end regionhaving a concave curvature with a maximal extremum at a third inflectionpoint.
 15. The method of claim 14, comprising stamping the singlecontinuous metallic body so that the minimal extremum extends verticallybeyond opposing ends of the curved body.
 16. The method of claim 14,comprising stamping the single continuous metallic body so that theopposing ends of the curved body point downward toward the substrate.17. The method of claim 14, comprising stamping the single continuousmetallic body so that each concave curvature has a smaller radius thanthe convex curvature.
 18. The method of claim 17, comprising stampingthe single continuous metallic body so that the radius of each concavecurvature is about ⅙ the radius of the convex curvature.